A Quadrature Amplitude Modulation (QAM) receiver is arranged for receiving a signal representing successive symbols, each described by an IQ pair with an In-phase (I) component and a Quadrature (Q) component. In a QAM digital receiver, the signal is initially converted from the analog domain to the digital domain through Analog to Digital Converters (ADCs) that sample each IQ pair, i.e. the respective I component and the respective Q component of each IQ pair.
In such QAM digital receivers, a timing recovery arrangement aims at determining the time instants at which the I and Q components are as largest, that is, determining an optimum time instant for sampling the IQ pair, to facilitate the detection of the symbols from the IQ sample pair.
Three methods of timing recovery are well known; Early-late gate algorithm, Mueller and Muller algorithm and Gardner algorithm.
Early-late gate algorithm requires three IQ sample pairs per symbol; two IQ sample pairs to determine the sampling timing error for performing timing recovery, and one IQ sample pair for the actual sampling for decoding purposes.
A problem is that for systems with very high symbol rates, it may be too expensive to use oversampling, i.e. sampling the each IQ pair more than once per symbol.
Mueller and Muller algorithm uses two IQ sample pairs per symbol. A problem with this algorithm is however that it works only after carrier recovery, which may be a drawback, as carrier recovery may be better performed on a timing recovered signal.
Gardner algorithm also uses two IQ sample pairs per symbol, and is probably the most common algorithm. However, a further problem is that in systems with very high speed symbol rates, even using only two IQ sample pairs per symbol for timing recovery may prove too costly and difficult.